Method of manufacture of a heart valve support frame

ABSTRACT

Methods for forming a support frame for flexible leaflet heart valves from a starting blank include converting a two-dimensional starting blank into the three-dimensional support frame. The material may be superelastic, such as NITINOL, and the method may include bending the 2-D blank into the 3-D form and shape setting it. A merely elastic material such as ELGILOY may be used and plastically deformed in stages, possibly accompanied by annealing, to obtain the 3-D shape. Alternatively, a tubular blank could be formed to define a non-tubular shape, typically conical. A method for calculating the precise 2-D blank shape is also disclosed. A mandrel assembly includes a mandrel and ring elements for pressing the blank against the external surface of the mandrel prior to shape setting.

RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.10/423,019, filed Apr. 24, 2003, now U.S. Pat No. 7,137,184 which is acontinuation of U.S. application Ser. No. 10/251,651, filed on Sep. 20,2002, now abandoned

FIELD OF THE INVENTION

The present invention relates generally to medical devices, and moreparticularly to a continuous three-dimensional support frame for use inheart valves, and methods and apparatuses for forming such supportframes.

BACKGROUND OF THE INVENTION

Two primary types of heart valve replacements or prostheses are known.One is a mechanical-type heart valve that uses a ball and cagearrangement or a pivoting mechanical closure to provide unidirectionalblood flow. The other is a tissue-type or “bioprosthetic” valve which isconstructed with natural tissue leaflets which function much like thosein a natural human heart valve; that is, the leaflets imitate thenatural action of the flexible leaflets that form commissures to sealagainst each other and ensure the one-way blood flow. In tissue valves,a whole xenograft valve (e.g., porcine) or a plurality of xenograftleaflets (e.g., bovine pericardium) provide the tissue leaflet occludingsurfaces that are mounted within a surrounding stent structure. Someattempts have been made to simulate such flexible leaflets with polymersand the like, and these designs can be grouped with bioprosthetic valvesfor the purpose of the present invention.

In most bioprosthetic-type valves, metallic or polymeric structureprovides base support for the flexible leaflets, which extend therefrom.One such support is an elastic “support frame,” sometimes called a“wireform” or “stent,” which has a plurality (typically three) of largeradius cusps supporting the cusp region of the leaflets of thebioprosthetic tissue (i.e., either a whole valve or three separateleaflets). The free ends of each two adjacent cusps converge somewhatasymptotically to form upstanding commissures that terminate in tips,each being curved in the opposite direction as the cusps, and having arelatively smaller radius. The support frame typically describes aconical tube with the commissure tips at the small diameter end. Thisprovides an undulating reference shape to which a fixed edge of eachleaflet attaches (via components such as fabric and sutures) much likethe natural fibrous skeleton in the aortic annulus.

The support frame is typically a non-ferromagnetic metal such as ELGILOY(a Co—Cr alloy) that possesses substantial elasticity. A common methodof forming metallic support frames is to bend a wire into a flat(two-dimensional) undulating pattern of the alternating cusps andcommissures, and then roll the flat pattern into a tube using acylindrical roller. The free ends of the resulting three-dimensionalshape, typically in the asymptotic region of the cusps, are thenfastened together using a tubular splice that is plastically crimpedaround the ends. See FIGS. 3 and 4 of U.S. Pat. No. 6,296,662 for asupport frame that is crimped together at a cusp midpoint. The plasticdeformation of the splice and wire ends therewithin may cause highresidual stresses, which can promote fatigue fracture of the supportframe, thus reducing the overall life of the heart valve. Further, theadded diameter of the splice may create an unsightly bulge at onelocation around the valve circumference that may interfere with theimplant process or provide a point of stress concentration.

Some valves include polymeric “support frames” rather than metallic, forvarious reasons. For example, U.S. Pat. No. 5,895,420 discloses aplastic support frame that degrades in the body over time. Despite somefavorable attributes of polymeric support frames, for example theability to mold the complex support frame shape, conventional metallicsupport frames are generally preferred for their elastic properties, andhave a proven track record in highly successfully heart valves. Forexample, the CARPENTIER-EDWARDS Porcine Heart Valve and PERIMOUNTPericardial Heart Valve available from Edwards Lifesciences LLC bothhave ELGILOY support frames and have together enjoyed the leadingworldwide market position since 1976.

What is needed then is an improved three-dimensional heart valve supportframe without the drawbacks of a conventional spliced support frame.Also needed is a simple and accurate method of manufacturing such asupport frame.

SUMMARY OF THE INVENTION

The present invention solves a number of drawbacks associated withconventional spliced support frames or wireforms in that a continuous,seamless length of material eliminates any non-uniformity in theperiphery of the frame. The material may be superelastic such that itprovides a highly flexible valve that can move with the dynamicpulsations of the surrounding cardiac tissue. In one particularly usefulembodiment, the valve is constructed so that it is implanted up theascending aorta and can expand and contract therewith. The presentinvention also provides several novel methods of forming support frames.

One such method of the present invention of forming an elastic materialinto a heart valve support frame includes providing a two-dimensionalblank of a continuous support frame in the elastic material, and formingthe two-dimensional blank into a continuous three-dimensional heartvalve support frame. A flat sheet of the elastic material may be usedand the two-dimensional blank separated therefrom, such as by cuttingthe elastic material along the perimeter of the two-dimensional blank.The elastic material may be a superelastic material. For example, thesuperelastic material is NITINOL and the step of forming includesheat-setting the NITINOL into the three-dimensional heart valve supportframe. Alternatively, the elastic material may be a conventional metaland the step of forming includes plastically deforming and thenannealing the metal.

The three-dimensional heart valve support frame may be formed byproviding a mandrel and conforming the two-dimensional blank over themandrel so that it assumes a three-dimensional shape matching theexterior shape of the mandrel. Once the blank is in the desired shape,the properties of the elastic material are altered while on the mandrelsuch that when it is removed from the mandrel it remains substantiallyin the three-dimensional shape corresponding to the heart valve supportframe. In one embodiment the heart valve support frame includes threecusps and three commissures and the two-dimensional blank accordinglyhas three cusp regions and three commissure regions. The step ofconforming the two-dimensional blank over the mandrel therefore includesorienting the two-dimensional blank over the mandrel by registering thecusp regions and commissure regions with a series of pins provided onthe mandrel, and pressing the two-dimensional blank flat against theexterior shape of the mandrel using at least one ring element.

The method may also include surface treating the three-dimensional heartvalve support frame to reduce features of high stress concentration. Thesurface treating can be by mechanical, chemical or electrochemicalprocess, such as electropolishing

A further aspect of the present invention is a method of calculating theshape of and making a heart valve support frame, comprising:

entering the desired shape of a heart valve support frame into a finiteelement program to obtain a three-dimensional support frame model;

simulating forces on the support frame model to cause it to assume atwo-dimensional pattern; and

using the shape of the two-dimensional pattern to form a two-dimensionalblank from a sheet of elastic material.

The method desirably includes forming the two-dimensional blank into athree-dimensional heart valve support frame by providing a mandrel andconforming the two-dimensional blank over the mandrel so that it assumesa three-dimensional shape matching the exterior shape of the mandrel. Ifthe elastic material is a superelastic material the method may includealtering the material properties of the blank while on the mandrel suchthat when it is removed from the mandrel it remains substantially in thethree-dimensional shape of the mandrel. The support frame model maydefine three arcuate cusps separated by three generally axially orientedcommissures, wherein the step of simulating forces involves simulatingthe application of generally axially oriented forces on the model suchthat the commissures rotate inward and the two-dimensional patternappears substantially like a three-leaf clover.

Prior to forming the two-dimensional blank, the two-dimensional patternmay be formed into a three-dimensional virtual support frame shapewithin the finite element program and then compared with the desiredshape of the heart valve support frame. If the virtual support frameshape does not match the desired shape of the heart valve support framethe two-dimensional pattern is adjusted.

Another aspect of the invention is a method of manufacturing a heartvalve, including providing a heart valve support frame blank made ofsuperelastic material and forcing the support frame blank intosubstantially the final shape of a heart valve support frame. Whilemaintaining the support frame blank in its substantially heart valvesupport frame shape the internal structure of the superelastic materialis altered so that it assumes that shape and forms the support frame.Finally, the heart valve is assembled by coupling a bioprosthetic valveor flexible leaflets to the support frame.

The method of manufacture may include providing a tube of thesuperelastic material. In this configuration, the heart valve supportframe blank is provided by separating a continuous support frame blankfrom the tube. The support frame blank is forced from its tubularconfiguration to other than a tubular configuration, such as conical,and then the internal structure of the superelastic material is altered.If the material is NITINOL the step of altering includes heat settingthe NITINOL.

Alternatively, the method of manufacture may include providing a flatsheet of the superelastic material. In that configuration, the heartvalve support frame blank is provided by separating a two-dimensionalblank of a continuous support frame from the flat sheet. Thetwo-dimensional blank is forced over a mandrel so that it assumes athree-dimensional shape matching the exterior shape of the mandrel.

Still further, the step of a slender wire of the superelastic materialmay be provided and bent over a mandrel so that it assumes athree-dimensional shape matching the exterior shape of the mandrel.

One further aspect of the invention is an intermediate apparatus in theformation of a prosthetic tissue-type heart valve comprising atwo-dimensional continuous (meaning seamless) heart valve support frameblank. The support frame blank is preferably NITINOL, and may have asquare cross-section. The tissue-type heart valve may be of the typehaving three leaflets, wherein the two-dimensional continuous supportframe blank includes three arcuate cusps and three commissurestherebetween in a pattern that resembles a three-leaf clover. Thecommissures each may have widened tips to increase their radius ofcurvature relative to commissures tips that have not been widened.

A still further aspect of the invention is a method of forming a heartvalve support frame, comprising providing a heart valve support frameand electropolishing the heart valve support frame. The heart valvesupport frame is desirably metallic and has a rectilinear cross-section.More preferably, the heart valve support frame is made of NITINOL. Themethod may further include, prior to electropolishing, first removingoxidation from the outer surface of the heart valve support frame. Oneway to removing oxidation is to microblast the outer surface of theheart valve support frame.

The step of electropolishing desirably involves submerging the heartvalve support frame into a conductive fluid bath, providing an anode anda cathode, and flowing a current between the anode and the cathode.Plate anodes may be positioned within the conductive fluid bath aroundthe heart valve support frame. The conductive fluid bath preferablycomprises nitric acid and methanol maintained at a temperature ofbetween about −28-32° C. In an exemplary embodiment, the conductivefluid bath is maintained at a conductivity level of between about140-190 μs at 23° C. and the current flows at a voltage of between about9-10 V.

The heart valve support frame may have an undulating shape with threecusps and three commissures and the method further includes within theconductive fluid bath, holding the heart valve support frame at threepoints around the undulating shape, and periodically repositioning theheart valve support frame during the electropolishing process. Forexample, the heart valve support frame may be rotated 120° twice from aninitial position.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view illustrating sub-assemblies of aprosthetic heart valve of the present invention;

FIG. 1A is a perspective view of a cloth-cover heart valve support frameof the present invention;

FIG. 1B is a sectional view through the support frame of FIG. 1A, takenalong line 1B-1B;

FIGS. 2A and 2B are elevational and top plan views, respectively, of anassembled prosthetic heart valve of the present invention;

FIG. 2C is an exploded perspective view of the components of the heartvalve of the present invention along with an implantation holder;

FIGS. 2D and 2E are perspective and elevational views, respectively, ofthe implantation holder attached to the prosthetic heart valve;

FIG. 3 is a plan view of one flexible leaflet suitable for use in theheart valve of FIG. 2A;

FIG. 4A is a schematic perspective line drawing of a two-dimensionalpattern of a heart valve support frame representing an intermediateproduct in the support frame manufacturing process;

FIG. 4B is a schematic perspective line drawing of a three-dimensionalpattern of the heart valve support frame formed from the two-dimensionalpattern of FIG. 4A;

FIG. 5 is a plan view of a two-dimensional heart valve support frameblank of the present invention;

FIG. 5A is a cross-sectional view of the support frame blank of FIG. 5,taken along line 5A-5A;

FIG. 6 is an exploded perspective view of a heart valve support frameand mandrel apparatus for forming thereof;

FIGS. 7A and 7B are elevational and plan views, respectively of theassembled mandrel apparatus and heart valve support frame;

FIG. 7C is a longitudinal cross-sectional view taken through the mandrelapparatus and support frame mounted thereon, and taken along line 7C-7C;

FIGS. 8A and 8B are plan and sectional views, respectively, of a lowerring element of the mandrel apparatus seen in FIGS. 6-7;

FIGS. 9A and 9B are plan and sectional views, respectively, of an upperring element of the mandrel apparatus seen in FIGS. 6-7;

FIG. 10 is a flowchart illustrating an exemplary heart valve supportframe manufacturing process; and

FIG. 11 is a flowchart illustrating an exemplary process for surfacetreating a heart valve support frame of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides an improved support frame, formingapparatus, and method as described herein and shown in the accompanyingdrawings.

The present invention pertains primarily to flexible leaflet heart valvesupport frames, which are also referred to in the art as stents orwireforms. As mentioned above, the flexible leaflets can be provided bya biological (i.e., xenograft) valve, biological leaflets, or syntheticleaflets. In this context, a “support frame” for a flexible leafletheart valve provides the primary internal structural support for theleaflets, and substantially mimics the natural fibrous skeleton of therespective valve annulus. More specifically, each of the leaflets has anouter edge that is coupled to a portion of the support frame such thatits inner edge is free to move within the orifice area of the valve,thus providing the opening and closing surfaces thereof. In the commonthree-leaflet prosthetic valves, the support frame has an undulatingshape with three arcuate cusps on the inflow end separated by threeupstanding and generally axially-oriented commissures on the outflowend. Around the circumference of the frame, the shape has an alternatingstructure of cusp-commissure-cusp-commissure-cusp-commissure, andgenerally describes a tubular surface of revolution about an axis. Somesupport frames describe a conical surface of revolution with the threecommissures on the outflow end of the valve being closer together thanthe three cusps. It should be understood, however, that the presentinvention is not limited to support frames for three-leaflet valves, andvalves with two or more than three leaflets may be constructed using thesupport frames embodied herein.

With reference now to FIG. 1, an exploded view of a number of componentsof an exemplary embodiment of a prosthetic heart valve 20 (shownassembled FIGS. 2A and 2B) of the present invention is shown. Forpurposes of discussion, the directions up and down, upper and lower, ortop and bottom, are used with reference to FIG. 1, but of course thevalve can be oriented in any direction both prior to and afterimplantation. From top to bottom, the heart valve 20 comprises a fabricsupport frame covering 22, a support frame 24, an implantation band 26,and a fabric covering 28 for the implantation band. Each of thecomponents seen in FIG. 1 is procured and assembled separately and thenjoined with the other subassemblies to form the fully assembled valve 20seen in FIG. 2A.

FIG. 1A illustrates a subassembly 25 of the support frame 24 and fabriccovering 22 thereover. The cross-section of FIG. 1B more clearly showsthe construction of the subassembly. For purpose of attaching to theremaining components, a flat or flange 30 is formed by additional fabricmaterial that has been sewn in two places 32 a, 32 b as shown.

FIG. 2C is an exploded view of the components of heart valve 20, inaddition to an implantation holder 34. The support frame/fabric coveringsubassembly 25 is shown above a group of three flexible leaflets 36,which, in turn, is above a subassembly 38 comprising the implantationband 26 and its fabric covering 28. These three subassemblies areattached together, such as by sewing, to result in the assembled valve20 as seen in FIGS. 2A and 2B. Finally, FIGS. 2D and 2E show the holder34 mounted on the valve 20. Although not shown, an elongated handle canbe attached to a threaded boss 40 on the holder for manipulating thevalve into its implant position.

Each of the components or subassemblies seen in FIGS. 1 and 2 includethree cusps separated by three commissures. For example, each of theleaflets 36 includes an arcuate lower cusp edge 42 terminating inupstanding commissure regions 44. Each leaflet 36 includes a coapting orfree edge 46 opposite the cusp edge 42. In the assembled valve 20, thecusp edges 42 and commissure regions 44 are secured around the peripheryof the valve, with the free edges 46 permitted to meet or “coapt” in themiddle. The support frame subassembly 25 also includes three cusps 50separated by three upstanding commissures 52. In like manner, theimplantation band 26 includes three cusp portions 54 separated by threeupstanding commissure portions 56. As seen in FIG. 2A, the assembledvalve 20 exhibits cusps 58 and commissures 60.

Further details of the sub-assemblies can be found in U.S. Pat. No.6,558,418, entitled FLEXIBLE HEART VALVE, filed on Jun. 14, 1999, whichdisclosure is expressly incorporated herein by reference. As describedin this earlier application, the implantation band subassembly 38 issewn or otherwise attached to the exterior of a further subassemblycomprising the group of leaflets 36 attached to the support framesubassembly 25. Outer margins of the implantation band subassembly 38extend outward from the rest of the valve and provide a platform throughwhich sutures can pass to attach the valve 20 to the patient's anatomy.

The heart valve 20 illustrated is designed to be attached not only atthe aortic annulus, but also up into the ascending aorta. The commissureportions 56 of the implantation band 26 are separated about a gap 62(see FIG. 1) extending substantially all the way up. Likewise, thesupport frame subassembly 25 is highly flexible such that the cusps 50generally pivot about the outstanding commissures 52. Ultimately, thecusps 58 of the valve 20, are surgically attached adjacent the patient'sannulus, while the upstanding commissures 60 are attached along theascending aorta. The high flexibility of the valve 20 permits relativemovement between these anatomical locations.

In an exemplary embodiment of the present invention, the internalsupport frame 22 of the subassembly 25 is made of a material that ishighly flexible so as to permit maximum relative movement between thevalve cusps 58 and commissures 60. That said, the support frame 22 mustpossess a minimum amount of stiffness to provide the desired support tothe leaflets 36. Therefore, there is a balance obtained between therequisite flexibility and stiffness.

The material for the internal support frame is desirably “elastic,”which means that it has a relatively high modulus of elasticity,preferably greater than or equal to 26 Msi. Polymers are generallyexcluded from this definition, although it is conceivable that specialformulations might function under these requirements. Various NITINOLalloys can also be suitable for making the internal support frame of thepresent invention as in certain circumstances they are considered to be“superelastic.” Other materials that maybe used include ELGILOY,titanium, stainless-steel, and similar expedients. These lattermaterials do not display superelasticity but are still elastic. Othermaterials may fit within this definition but they must be suitable forlong-term implantation in the body.

The term “superelastic” (sometimes “pseudoelastic”) refers to thatproperty of some materials to undergo extreme strains (up to 8%) withoutreaching their failure stress limit. Some so-called shape memory alloys(SMAs) are known to display a superelastic phenomena or rubber-likebehavior in which a strain attained beyond the elastic limit of the SMAmaterial during loading is recovered during unloading. This superelasticphenomenon occurs when load is applied to an austenitic SMA articlewhich first deforms elastically up to the yield point of the SMAmaterial (sometimes referred to as the critical stress). Upon thefurther imposition of load, the SMA material begins to transform intostress-induced martensite or “SIM.” This transformation takes place atessentially constant stress, up to the point where the SMA material iscompletely transformed into martensite. When the stress is removed, theSMA material will revert back into austenite and the article will returnto its original, pre-programmed programmed or memorized shape.

FIG. 3 illustrates one of the leaflets 36 in plan view. As mentioned,the leaflet 36 has an arcuate cusp edge 42 a pair of commissure regions44, and a free edge 46. The free edge 46 can be non-linear; e.g., thefree edge in FIG. 3 increases to a flat portion 47 in the centerthereof.

FIGS. 4A and 4B are schematic perspective line drawings of stages in themanufacturing process of a heart valve support frame of the presentinvention. FIG. 4A illustrates a two-dimensional pattern 80 of thesupport frame representing an intermediate apparatus in themanufacturing process. When viewed from above, the pattern 80 resemblesa three-leaf clover with three outwardly extending generally circularlobes or cusps 82 a, 82 b, 82 c separated by three inwardly directedcommissures 84 a, 84 b, 84 c. The cusps 82 and commissures 84 are evenlydistributed 120° apart about a central axis 86. FIG. 4B is athree-dimensional pattern 90 of the support frame that is made bystarting with the two-dimensional pattern 80. Generally, the commissures84 have been rotated upward and outward from their position in thepattern 80 of FIG. 4A to their position in the pattern 90 of FIG. 4B.The resulting three-dimensional pattern 90 has the undulating shapedescribed above for heart valve support frames in that the cusps 82extend in one direction along axis 86 while the commissures 84 extend inthe opposite direction. Furthermore, the pattern 90 desirably describesa conical surface of revolution. Of particular note is that the pattern90 includes no seam or splice that might otherwise provide a stressconcentration point in the finished support frame.

With reference now to FIG. 5, a two-dimensional support frame blank 100is shown in plan view, illustrating the aforementioned three-leaf cloverpattern. As before, there are three generally circular cusps 102 a, 102b, 102 c separated by three inwardly directed commissures 104 a, 104 b,and 104 c. Each commissure 104 is formed by a pair of generallyasymptotic regions 106 projecting from the adjacent cusps 102 and a tipregion 108. The asymptotic regions 106 converge until they are separatedby a narrow gap G just outward from the tip region 108. The tip region108 is desirably wider than the gap G, and thus has a greater radius ofcurvature and is more flexible than the tip would be if the asymptoticregions 106 were simply joined by a round section. The widened tipregion 108 helps to prevent the commissures 104 from piercing any fabriccovering attached thereover in the assembled valve.

FIG. 5A illustrates a cross-section through one of the cusps 102 of thesupport frame blank 100. This embodiment is square with rounded corners,preferably from electro-polishing, although other configurations arecontemplated. The cross-sectional thickness, given as t in FIG. 5, isdesirably relatively slender, preferably between about 0.46-0.76 mm(0.018-0.030 inches). More preferably, the thickness is about 0.66 mm(0.026 inch). If the cross-section is circular (or other shape) thediameter (or effective diameter) would be within the same range.

Methods of Manufacture

The following explains a preferred sequence of manufacturing steps toresult in the two-dimensional blank 100 of FIG. 5, and then athree-dimensional support frame. This manufacturing sequence isspecifically designed for support frame blank 100 made of superelasticmaterial, preferably NITINOL, and can be seen in the flowchart of FIG.10. A similar process for conventional, merely elastic metals will bealso described below. FIG. 10 is an overview of an entire exemplarymanufacturing process in flowchart form.

Initially, a flat sheet of elastic material is procured and cleaned. Thetwo-dimensional support frame blank 100 is then separated from thesurrounding sheet of material by photo/chemical etching, laser cutting,electric discharge machining, or similar processes. The particular shapeof the blank 100 is obtained with knowledge of the end productthree-dimensional support frame, as will be described below. At thisstage, the cross-section of the blank 100 is either square orrectangular with relatively sharp corners. For the sake of producinguniform stresses when cold working the blank 100, the cross-section isdesirably square.

The blank 100 is then forced into approximately the final shape of theheart valve support frame by, for example, fitting it over a mandrel 110as seen in FIGS. 6 and 7A-7C. The mandrel 110 has an exterior conicalbody 112 with a rounded nose 114. The mandrel 110 further includes adisk-like base 116, a first set of three pins 118 closely adjacent thebase, a second set of three pins 120 positioned farther away from thebase, and a set of commissure orientation pins 122 located even fartherfrom the base. These pins can best be seen in cross-section FIG. 7C.

Although not shown, it will be appreciated by the reader that by takingthe two-dimensional blank 100 of FIG. 5 and forcing it over the nose114, the commissures 104 are forced upward and outward, such that theblank assumes the three-dimensional shape 126 seen in FIG. 6. Thecommissures 104 are registered with the set of commissure orientationpins 122. The superelasticity of the material permits this extremedeformation.

Next, a first ring element 130 (FIGS. 8A and 8B) is lowered over themandrel 110 until a series of cusp pins 132 contact the base 116. Thering element 130 further includes three cutouts 134 which facilitatepassage over the sets of pins 118, 120, and 122, and also orient thering element. The inner diameter of the ring element 130 is sizedslightly larger than the outer diameter of the conical body 112 at itslowest end such that the ring element forces the three-dimensional blank126 against the exterior of the mandrel body. A second ring element 140(FIGS. 9A and 9B) more effectively presses the three-dimensional blank126 against the mandrel body 112. That is, the second ring element 140passes over the mandrel nose 114 until a series of cutouts 142 registerwith the commissure orientation pins 122. The final assembly in thismanufacturing step is seen in cross section in FIG. 7C. The second ringelement 140 descends until it rests on the second series of pins 120, asseen in FIG. 7C, at which point the inner diameter presses thethree-dimensional blank 126 against the mandrel body 112. Although tworing elements are shown and described, a single element may also beused.

Once the three-dimensional blank 126 is forced to assume the exteriorshape of the mandrel 110 it is set into that shape, preferably byheating. A superelastic alloy such as NITINOL can withstand a relativelylarge magnitude of strain without deforming, such as when converting thetwo-dimensional blank 100 into the three-dimensional blank 126, but itwill spring back into the original shape unless set to that modifiedshape. Consequently, heat is applied to the blank 126 when it ismaintained over the mandrel 110 resulting in a shape set. Theparticulars of this operation will not be explained in exhaustive detailother than to say that the particular temperature and time of the shapesetting operation depend on the alloy composition, configuration of theworkpiece, and history including the degree of cold work alreadyassociated with the material. An exemplary embodiment involves a NITINOLalloy having an atomic composition of about 50-50 Nickel (Ni) toTitanium (Ti), which corresponds to between about 55-57% Ni (preferablyabout 56%) by weight, between about 43-45% Ti (preferably about 54%) byweight, and trace elements such as Carbon and Oxygen. It should be notedthat the presence of Oxygen and Carbon should each be limited to lessthan 500 ppm to help avoid brittleness and ensure an adequate fatiguelife of the final support frame. The shape set is done at 560° C.±38° C.(1040° F.±100° F.) for 4.0±2.0 minutes. Even then, once removed from themandrel assembly, the blank 126 will spring slightly outward and takethe form of the final heart valve support frame.

As mentioned above, a different process will be used for moreconventional metals such as ELGILOY or titanium. These metals are notsuperelastic, and thus will not withstand the strains associated withdirectly converting the two-dimensional form to the three-dimensionalform. Therefore, after separating the two-dimensional pattern from thesheet, the final form of the support frame is reached by gradually(i.e., in stages) plastically-deforming the blank. After eachdeformation step the blank is annealed to remove residual stresses byapplying heat for a particular amount of time. The tools used to bendsuch blanks are not shown in present application, but one of skill inthe art will understand that they take form of interpolated shapesbetween the mandrel 110 and a mandrel with a much shallower conicalangle. Although this process is relatively straightforward, theaforementioned single step formation of a superelastic support frame ispreferred.

Another way to obtain the final heart valve support frame is to startwith a tube of elastic or superelastic material and separate acontinuous support frame blank therefrom using the aforementioned means(e.g., laser cutting). The support frame is then modified into anon-tubular shape, so as to describe a conical surface of revolution,for example. The modification of a superelastic material such as NITINOLis accomplished by holding the blank into the desired shape and settingthat shape with heat, for example. For a elastic material such asELGILOY, the blank would be plastically deformed into the desired shapeand annealed to remove residual stresses. A mandrel such as describedand shown previously could be used to define the conical support frameshape.

FIG. 10 illustrates an exemplary heart valve support frame manufacturingprocess that involves converting a 2-dimensional blank into a 3-Dsupport frame. After the three-dimensional support frame has been formedinto shape it is subjected to surface treatment to round the edges.Mechanical, chemical or electrochemical processes can be used, includingtumbling, corner grinding, chemical etching, or microblasting andelectropolishing as shown. Sometimes two or more of these processes areused in conjunction with each other to obtain the desired finish. Forexample, tumbling and electropolishing can be used together to roundcorners as well as get the desired surface finish. Rounding of thecorners of the cross-section minimizes potential sites for stressconcentration. The resulting structure is a single piece, continuousheart valve support frame that has rounded corners and a smooth surface.

FIG. 11 is a flowchart describing one particular sequence of events in asurface treating process of the present invention. The goal of theprocess is to reduce surface defects and roughness to reduce thestresses associated with prolonged use in the body. That is, the supportframe is subjected to millions of cycles of systolic-diastolic movement,and its fatigue life is a large concern.

The process begins with the formation of the 3D support frame asdescribed above. To ensure uniform treatment, the frame is mounted orheld on a support at three points, preferably along the asymptoticregions 106 as seen in FIG. 5, though other configurations are possible.As mentioned above and seen in FIG. 10, the frame has previously beenmicroblasted to remove oxidation and permit proper conduction to theexterior surface. The frame is submerged in a conductive fluid bath ofchemical, for example nitric acid and methanol. In one embodiment, theconductivity of a nitric acid and methanol fluid bath desirably rangesbetween about 140-190 μs at 23° C. and is monitored periodically tomaintain this range. The process desirably occurs at a much lowertemperature but this calibration at about room temperature correlates tothe proper conductivity at operating temperatures.

There are a number of control variables for the electropolishing processother than the bath conductivity, including the temperature (desirablyabout −30° C.±2° C.), voltage (desirably between 9-10 V, and morepreferably about 9.5 V), current density, agitation of the fluid in thebath, anode and cathode configurations and distance, frame holdingarrangement, and others incidental to the process. Desirably thestainless steel bath tank itself provides the cathode and a pair ofplates on either side of the frame provide the anodes. However, otherarrangements are feasible. It has also been found desirable to rotatethe frame three times during the electropolishing to ensure symmetricmaterial removal.

One of the tasks in setting up a manufacturing process as describedherein involves carefully calculating the shape of the two-dimensionalblank, which will then be bent into the three-dimensional form. One wayto perform this calculation involves utilizing a finite element program.Such programs are common in the design industry and generally simulateor model real workpieces and their response to simulated forces anddeflections.

To begin, a finite element model of the final configuration of thesupport frame is input into the program. The frame is modeled withcircular cross-section beam elements. Next, vertical displacements areimposed on all the nodes between the beam elements to bring the framemodel into a plane. The other degrees of freedom are not constrained toguarantee that the flat pattern will not be carrying any extra load. Themodel is then updated with the appropriate cross-section, such assquare, and the stress in the two-dimensional pattern is canceled. Theintent is to reproduce the pattern “as cut” from a sheet of material.

In the finite element program, the two-dimensional pattern is mountedaround a simulated mandrel, much as described above. Contact detectionis enabled between the frame and mandrel, and displacements are imposedto the cusp nodes. This simulates the frame being pulled down over themandrel. Mounting of the first ring element is simulated by imposingdisplacements on the cusp nodes of the frame to bring them in contactwith mandrel. The shape of the model is then compared to the shape ofthe final support frame. At this point, only the cusp and commissurenodes match and a determination is made where the model is farthest wayfrom the mandrel. Once done, placement of a second ring element issimulated at the location where the frame is farthest way from themandrel. This is normally sufficient to anchor the frame model firmlyagainst the mandrel model.

Finally, a comparison between the frame model and the desired supportframe shape is made along the whole frame. If there is a mismatch, thegeometry of the initial two-dimensional pattern is adjusted accordingly,and a process is repeated until the correct shape is obtained.

It will be appreciated that the invention has been described hereabovewith reference to certain examples or preferred embodiments as shown inthe drawings. Various additions, deletions, changes and alterations maybe made to the above-described embodiments and examples, and it isintended that all such additions, deletions, changes and alterations beincluded within the scope of the following claims.

1. A method of forming a heart valve support frame, comprising:providing a heart valve support frame having an undulating shape withthree cusps and three commissures; and electropolishing the heart valvesupport frame by twice rotating the heart valve support frame 120° froman initial position during the electropolishing process to ensuresymmetric material removal.
 2. The method of claim 1, wherein the heartvalve support frame is metallic and has a rectilinear cross-section. 3.The method of claim 1, wherein the heart valve support frame is made ofNITINOL.
 4. The method of claim 1, further including: prior toelectropolshing, first removing oxidation from an outer surface of theheart valve support frame.
 5. The method of claim 4, wherein the step ofremoving oxidation comprises microblasting the outer surface of theheart valve support frame.
 6. The method of claim 1, wherein the step ofelectropolishing comprises: submerging the heart valve support frameinto a conductive fluid bath; providing an anode and a cathode; andflowing a current between the anode and the cathode to electropolish theheart valve support frame.
 7. The method of claim 6, wherein the step ofproviding an anode comprises positioning plate anodes within theconductive fluid bath around the heart valve support frame.
 8. Themethod of claim 6, wherein the conductive fluid bath comprises nitricacid and methanol.
 9. The method of claim 6, wherein the conductivefluid bath is maintained at a temperature of between about −28 to −32°C.
 10. The method of claim 6, wherein the conductive fluid bath ismaintained at a conductivity level of between about 140-190 μs at 23° C.11. The method of claim 6, wherein the current flows at a voltage ofbetween about 9-10 V.
 12. The method of claim 1, wherein each commissureis formed by a pair of generally asymptotic regions projecting from theadjacent cusps and a tip region, the method further comprising holdingthe heart valve support frame at three of the asymptotic regions.
 13. Amethod of forming a heart valve support frame, comprising: providing aheart valve support frame having an undulating shape with three cuspsseparated by three commissures; submerging the heart valve support frameinto a conductive fluid bath; within the conductive fluid bath, holdingthe heart valve support frame at discrete points around the undulatingshape; providing an anode and a cathode and flowing a current betweenthe anode and the cathode to electropolish the heart valve support framewithin the conductive fluid bath; and twice rotating the heart valvesupport frame 120° from an initial position during the electropolishingprocess to ensure symmetric material removal.
 14. The method of claim13, wherein the heart valve support frame is made of NITINOL.
 15. Themethod of claim 13, wherein the step of providing an anode comprisespositioning plate anodes within the conductive fluid bath around theheart valve support frame.
 16. The method of claim 13, furthercomprising: holding the heart valve support frame at just three pointsaround the undulating shape.
 17. The method of claim 13, wherein eachcommissure is formed by a pair of generally asymptotic regionsprojecting from the adjacent cusps and a tip region, the method furthercomprising: holding the heart valve support frame at three of theasymptotic regions.